The present invention relates to a process for producing silicon carbide heating elements having improved strength, low electrical resistivity and a low negative temperature coefficient of resistance.
Silicon carbide heating elements have for sometime been used as heating elements for electric furnaces, and more recently, they have been used as heating elements for various industrial instruments. For such uses, the silicon carbide is required to have high strength, low electrical resistivity and a low negative temperature coefficient of resistance. Among processes developed recently for sintering SiC, a few processes are the following:
(1) Japanese Patent Application (OPI) No. 78609/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), entitled "Process For Producing Silicon Carbide Ceramics Having High Density", describes a sintered material having a density of more than 95%, based on the theoretical density, obtained by sintering a mixture of SiC, a boron compound and a carbonaceous material at 2050.degree. C. in Ar for 60 minutes. The theoretical density hereinafter refers to a density of SiC single crystal. However, it is not suitable as a heating element, because the electrical resistivity is too high at room temperature and too low at a high temperature (for example, 500.degree. to 1500.degree. C.).
If sintering is carried out in nitrogen at atmospheric pressure from the beginning at less than 2200.degree. C., though the electrical resistivity is less than 1.0 .OMEGA.-cm, the density is only about 80% or so based on the theoretical density, and the strength is low. Furthermore, if the sintering is carried out at a higher temperature, though the density increases to about 90% based on the theoretical density, the electrical resistivity becomes as large as from 10 to 10.sup.6 .OMEGA.-cm, and is lacking in stability.
(2) Japanese Patent Application (OPI) No. 110499/77, entitled "Fuel Igniter Comprising A Novel Silicon Carbide Composition And Process For Preparing The Composition", describes a process comprising sintering a powder comprising more than 95% SiC by hot pressing to a density of 2.5 g/cc, and doping with N, P, As, Sb, or Bi as solid phase or a vapour phase (that is, the N, P, As, Sb, or Bi can be introduced by simply mixed solids containing the noted elements or, by permeation of vapour of the noted element) to obtain the electrical resistivity of 0.66 .OMEGA.-cm at room temperture. However, the product is not suitable as a power economization type heating element, because the electrical resistivity at 1350.degree. C. is 0.12 .OMEGA.-cm and the temperature coefficient of resistance is inferior.
(3) Japanese Patent Application (OPI) No. 121810/78, entitled "Sintered Silicon Carbide Having High Density And High Thermal Impact Resistance", describes a product having a density of at least 85%, based on the theoretical density, produced by adding from 0.3 to 3.0% by weight of BN, BP or AlB.sub.2 as an additive and furthermore from 150 to 500% by weight of C, based on the boron content of the boron additive, and sintering the resulting mixture at from 1900.degree. to 2500.degree. C., which is substantially equal in the sintering processes and the physical or electrical properties of the resulting sintered material to a sintered material obtained at an atmospheric pressure described in Japanese Patent Application (OPI) No. 78609/75. In such a sintered material having a density of 95% or more based on the theoretical density, there is a fault that oxidation resistance is somewhat inferior to that of B or B.sub.4 C added type materials.
Thus, there has been proposed in Japanese Patent Application No. 72464/79 (corresponding to U.S. Ser. No. 157,738), now U.S. Pat. No. 4,336,216 entitled "Process For Producing Silicon Carbide Heating Elements", a process for producing silicon carbide heating elements which comprises adding boron or a boron compound in an amount corresponding to 0.3 to 3.0% by weight as boron and carbon or a carbon compound in an amount corresponding to 0.1 to 3.0% by weight as carbon to a SiC powder having 1.0.mu. or less of the average particle size, blending and molding the mixture, carrying out primary sintering in vacuum or in an inert atmosphere, except nitrogen, to obtain a density of from 70 to 95% based on the theoretical density, and thereafter carrying out secondary sintering at from 1600.degree. to 2200.degree. C. in a nitrogen atmosphere to produce a silicon carbide heating element having a density of at least 80% based on the theoretical density and an electrical resistivity of 1.0 .OMEGA.-cm or less. The resulting material has high strength because of having high density and has low resistance, and the electrical resistivity at a high temperature thereof does not decrease significantly.